1. Field of the Invention
The present invention relates to a high-speed motor/generator drivable by a turbine, and more particularly to a high-speed motor/generator mounted on the shaft of a turbine which can be driven by the energy of an exhaust gas emitted from an internal combustion engine.
2 The Prior Art
Turbochargers incorporated in some internal combustion engines operate by driving a turbine utilizing the energy of an exhaust gas discharged by an internal combustion engine, and charging air into engine cylinders by a compressor coupled to the turbine for efficiently burning fuel in the engine cylinders. Japanese Laid-Open Utility Model Publication No. 58(1983)-6925 discloses a turbocharger system for an internal combustion engine, in which an ac machine is directly coupled to the rotatable shaft of a turbocharger. When an increased torque is produced by the engine or the engine is operated for a higher torque, the AC machine is operated as a motor, and when a reduced torque is produced by the engine or the engine is operated for a lower torque, the AC machine is operated as a generator to control the frequency of the current.
Generally used high-speed generators have a limit speed of a few tens of thousand revolutions per minute. If a double-pole rotary machine rotates at a speed of thirty thousand revolutions per minute, for example, the iron core of the rotary machine is placed in an alternating field having a frequency of 500 Hz. Even where the iron core is made up of silicon steel plates, any iron loss thereof is small, and substantially no problem arises.
However, when a machine rotates at a speed ranging from one hundred thousand revolutions per minute to two hundred thousand revolutions per minute, such as an exhaust gas turbine, the alternating field produced by a rotary machine directly coupled thereto has a frequency of 1000 Hz or higher. If the iron core of such a rotary machine is constructed of silicon steel plates, then its iron loss is greatly increased to produce a heat loss, resulting in a sharp reduction in the efficiency of the rotary machine.